Methods of producing phosphorus esters of polyol alkanoic acids



United States Patent 3,215,720 METHODS OF PRODUCING PHOSPHORUS ESTERS 0F POLYOL ALKANOIC ACIDS James D. Atwood and Ronald H. Wile, Point Pleasant, W. Va., and Paul C. Pearson, Summit, NJ., assiguors to Celanese Corporation of America, New York, N.Y., a corporation of Delaware No Drawing. Filed Jan. 3, 1961, Ser. No. 80,046 Claims. (Cl. 260461) This invention relates to synthetic lubricants and more particularly to ester lubricants adapted to be subjected to elevated temperatures for extended periods. This in vention also relates to an improved method for making such ester lubricants.

It is known that esters of polyols with alkanoic acids are useful as lubricants and that esters of polyols, such as trimethylolpropane, trimethylolethane and pentaerythritol which have at least two methylol groups bonded to a single quaternary carbon atom are particularly useful for high temperature stability. However, despite the enhanced thermal stability of such esters, they have not been able to meet the extremely rigorous oxidative stability demands of certain applications such as in the lubrication of high performance jet turbine aircraft engines.

In addition, the yield of such esters, based upon the percentage of desired ester produced upon consumption of the feed, has been of the order of 87 to 89%, resulting in the waste of a substantial amount of raw material.

' It is an object of this invention to produce a polyol ester of exceptional oxidative stability by a process which permits the recovery of at least 93% of said ester from the feed consumed.

In accordance with one aspect of this invention a method is provided for preparing an ester of at least one alkanoic acid having an average chain length between 4 and 12 carbon atoms and a polyol having a quaternary carbon atom bonded to at least two methylol groups which comprises reacting said alkanoic acid and said polyol in a reaction mixture containing a compound having the structure:

wherein the free oxygen valences are attached to members of the group consisting of hydrogen atoms, alkyl groups, alkylene groups and aryl groups, of which no more than two oxygen atoms are attached to aryl groups.

The compounds defined above, include phosphoric acid, partial alkyl esters of phosphoric acid, and trialkyl phosphates, alkylene alkyl phosphates, alkylene bis(dialkyl phosphates), aryl dialkyl phosphates and diaryl alkyl phosphates. Trialkyl phosphates are preferred.

Among the specific compounds which may be used are phosphoric acid, butyl phosphoric acid, dipropyl phosphoric acid, tripropyl phosphate, tributyl phosphate, triamyl phosphate, phenyl dibuty phosphate, diphenyl methyl phosphate, ethylene butyl phosphate and ethylene bis (dibutyl phosphate).

Trialkyl esters wherein the alkyl groups have from 1 to 10 atoms atoms, and particularly tributyl phosphate are especially preferred. Usually, about 0.2 to 1.0 weight percent of phosphate compound, based on the theoretical weight of ester formed, constitutes a suitable range of proportions, with from 0.3 to 0.6 weight percent being preferred.

The alkanoic acids used in accordance with this invention are pure compounds or mixtures of compounds having average chain lengths between 4 and 12 carbon ice atoms and preferably between 5 and 9 carbon atoms. The individual acids may range in chain length from 2 to 18 carbon atoms. Normal acids are preferred, although branched alkanoic acids may also be used, particularly those with no more than two carbon atoms in side chains. With trimethylolpropane as the polyol, acids having from 5 to 9 carbon atoms are generally used, with pure heptanoic acid being especially preferred.

The polyols used are those having at least two, and preferably three methyol groups on a quaternary carbon atom. Among the polyols which may be used are trimethylolpropane, trimethylolethane, neopentyl glycol, pentaerythritol, 2-butyl-2-ethy1-1,3-propanediol and 2,2,4- trimethyl-1,3-pentanediol.

The general procedure for the preparation of the esters of this invention is based on the reaction of the polyol with a slight excess of alkonoic acid with the removal of water by volatilization, preferably in the presence of an azeotroping agent, such as a liquid hydrocarbon.

With trimethylolpropane as the polyol, for example, slightly more than 3 moles of alkanoic acid are used for each mole of polyol, say between about 3.3 and 3.6 moles. From about 2 to about 12 parts by weight of an azeotroping agent, such as xylene, are added per parts of reactant mixture. The azeotroping agent is preferably a hydrocarbon boiling between 80 and 150 C. Among the preferred azeotroping agents in addition to xylene are toluene, cyclohexane, benzene and mixtures of aliphatic hydrocarbons, such as petroleum naphtha.

The esterification reaction is generally carried out at atmospheric pressure, under reflux conditions at a temperature between about and 350 C. The water is collected overhead as it is formed, together with the xylene azeotroping agent. The reaction is continued, until the hydroxyl content of the reaction mixture is not higher than 0.1% by weight, generally between about 4 and 20 hours.

After completion of the esterification reaction, the pressure is lowered to about 5 to 10 mm. of mercury, absolute and the product is stripped at a temperature up to about 240 C. to remove the xylene and excess acid. The acid content should be reduced to not more than 2% by weight in order to avoid emulsion formation during the subsequent washing steps.

After stripping, the ester is washed with caustic solution and then with sodium chloride solution and finally vacuum stripped to remove water. Other methods of reducing the acid content may be used, such as washing with sodium carbonate solution, stirring with an aqueous slurry of an alkaline material, such as calcium oxide, sodium or potassium carbonate or treatment with an ion exchange resin.

The dehydrated product is then treated with decolorising charcoal and finally filtered through diatomaceous earth.

The tributyl phosphate, or other phosphate compound must be present during at least a portion of the esterification reaction, but preferably is present throughout the reaction period. Tributyl phosphate added to a finished ester does not enhance its oxidative stability. T-riaryl phosphates, such as tricresyl phosphate, are not effective whether added to the finished ester or to the reaction.

While it is not desired to be bound by any particular theory of operation, it is believed that the tributyl phosphate per se does not act as a stabilizing compound but that it reacts in the esterification system to form a product which has a stabilizing action. It is known that the finished ester, even after distillation and other purification procedures contains about one-half of the phosphorus originally in the reaction zone. This phosphorus is apparently molecularly bound into compounds boiling within about 20 C. at 1 mm. of mercury above the boiling 3 point of the polyol ester, or between 230 C. and 250 C. at 1 mm. of mercury for trimethylolpropane heptanoate.

4 Viscosity (cs. at 100 F.)

The lowered yields of the prior art processes or prep- Time (hours) aration are also believed to be overcome by the stabilizing Sample action of the phosphate reaction products. It is believed 5 25 0 35 that all of the polyol consumed in the esterification reaction in the presence of excess acid is converted to the tri- 15.78 31.80 ester, but that a portion of the triester is degraded dur- 15-90 17-39 17-49 19-09 ing the esterification and subsequent stripping due to the l severe conditions used. The presence of tributyl phos- 10 Acldlty 8 KOH P 8 phate in the esterification zone is believed to lead to the formation of a stabilizing reaction product which sharply Time (hours) reduces degradation and permits the recovery of more of Sample the desired product. 0 25 30 35 The esters produced in accordance with this invention 15 may be used as a base stock for high temperature lubri- Ia 0.02 2. 49 cants and may be blended with additives known to con- 1b tribute to greater thermal and oxidative stability and improved lubricity, to provide metal deactivation and to ar- Make'up'rellacemem f Volatlles lost rest lead corrosion. 20

A particularly useful anti-oxidant additive system in- Time cludes from 0.5 to 3 weight percent of a diaromatic secsample ondary amine, such as phenyl a-naphthylamine and from 5 0.5 to 3 weight percent of a phenosilazine compound, such as N-ethyl-phenodiphenyl silazine, as described in the 25 Ia 11,0 copending application of John Koch, Jr., Serial No. 808,- lb 1L5 filed April 23, 1959, now US. Patent No. 3,036, y f total Volatiles g KOH p g EXAMPLE I 30 Time (hours) (a) Trimethylolpropane (250 parts by weight) and Sample heptanoic acid (800 parts by weight) were charged to a 5 10 15 0 25 30 35 still equipped with a 5 plate column and heated under nitrogen until reflux. Naphtha (120 parts by weight of g 3% boiling range 115-130 C.) was added and the mixture 35 was refluxed for about four hours until the water removal was complete. The reaction was continued for an EXAMPLE In additional hour at a pot temperature of 280 C. and a T i th l l (335 parts b i ht), heptanoic Slight Superatmosphefie Pressure P -gJ- The acid (1072.5 parts by weight) and phosphoric acid (7.0 mixture was then stripped of naphtha and excess acid and 40 parts b i h were i t i d at a temperature f Washed with 3% aqueous Sodium Carbonate The Oil 240 C. for 22 hours using xylene as a water entrainer. layer was Separated, dehydrated at 100-105 and The reactor product was stripped to a temperature of mm. HgA then carbon treated and filtered. The yield 230 C, 1 HgA,lCQO1ed d h d 5 ti with of ester was 870% based on the Y P p 1% aqueous sodium carbonate and twice with water. The procedure of was repeated, except that The product was dehydrated at 100 C. and 1 mm. HgA, 4 parts by weight of tributyl phosphate was present in tr t d ith carbon d fi1t d the initial reactant mixture. The yield in this case was Th above ester was bl d d ith 1% b i h f 96.5%, based on the trimethylolpropane. phenyl-a-naphthylamine and 1% by weight of N-ethylphenodiphenyl silazine. The blend was subjected to an EXAMPLE H oxidation stability test as described above. The results Each of the esters produced in Example I was blended were as follows:

Hours Test Units Viscosity Cs. at 100 F 16.01 19. 42 19. 40 18.99 19.51 Acidity Mgs. KOH/gm 0.49 0.33 0. 43 0. 47 0. 43 Make-up Grams 8 10 10 B Total overhead acidity..- Mgs. KOH/gm 5. 8

with 1% by weight of phenyl-a-naphthylamine and 1% by weight of N-ethyl-phenodiphenyl silazine. The blends were subjected to oxidation stability tests in a six hole aluminum block equipped with a thermostat and heater. The tests were run in accordance with Revised Mil-L- 9236A with 96 liters of air per hour passing through the 250 ml. samples maintained at 425 F. Volatile material stripped from the samples during the run was replaced at 5 hour intervals by an equal volume of fresh sample. The tests were continued until a sharp increase in viscosity, acidity and volatile losses showed breakdown of the sample. The results were as follows:

It is to be understood that the foregoing detailed description is given merely by way of illustration and that many variations may be made therein without departing from the spirit of our invention.

Having described our invention what we desire to secure by Letters Patent is:

1. An ester of at least one alkanoic acid having an average chain length between 4 and 12 carbon atoms and a polyol having up to about 9 carbon atoms, at least one of said carbon atoms being a quaternary carbon atom bonded to at least two methylol groups and up to 2 lower alkyl groups, said ester containing a minor amount of the product produced by the reaction of said polyol and said alkanoic acid under esterification conditions with a substantially neutral compound having the structure:

H/ P O' O wherein the free oxygen valences are attached to members of the group consisting of alkyl, alkylene and aryl free from substituents which are reactive in this reaction mixture, of which no more than two oxygen atoms are attached to aryl.

2. A trimethylolpropane trialkanoate having alkanoate chains formed from at least one alkanoic acid having an average chain length of to 9 carbons atoms, said trialkanoate containing a minor amount of the product produced by the reaction of trimethylolpropane and said alkanoic acid under esterification conditions with a substantially neutral compound having .the structure:

wherein the free oxygen valences are attached to members of the group consisting of alkyl, alkylene and phenyl, of which no more than two oxygen atoms are attached to phenyl.

3. An ester of at least one alkanoic acid having an average chain length between 4 and 12 carbon atoms and a polyol having a quaternary carbon atom bonded to at least two methylol and up to a lower alkyl, said ester containing a minor amount of phosphorus-containing compounds boiling within about 20 C. at 1 mm. of mercury above the boiling point of said polyol ester said phosphours-con-taining compounds being the reaction product of said polyol, said acid and at least one compound selected from the group consisting of phosphoric acid, partial esters of phosphoric acid and phosphoric acid triesters having at least one free oxygen valence satisfied by a residue selected from the group consisting of alkyl and alkylene residues.

4. A trimethylolpropane heptanoate containing phosphorus-containing compounds boiling between 230 C. and 250 C. at 1 mm. of mercury said phosphorus-containing compounds being the reaction product of at least one polyol selected from the group consisting of trimethylolpropane, trirnethylolethane, propentyl glycol, pentaerythritol, 2-butyl-2-ethyl-1,3-propanediol, and 2,2, 4-trimethyl-1,3-pentanediol; an alkanoic acid containing about 4 to 12 carbon atoms; and at least one compound selected from the group consisting of phosphoric acid, butyl phosphoric acid, dipropyl phosphoric acid, tripropyl phosphate, tributyl phosphate, triamyl phosphate, phenyl dibutyl phosphate, diphenyl methyl phosphate, ethylene butyl phosphate and ethylene bis (dibutyl phos phate).

5. Method of preparing an ester of at least one alkanoic acid having an average chain length between 4 and 12 carbon atoms and a polyol having up to about 9 carbon atoms and one of said carbon atoms being a quaternary carbon atom bonded to at least two methylol, said polyol being free from constituents other than said Ihydroxyls which are reactive in this reaction mixture, which comprises reacting said alkanoic acid and said polyol in a reaction mixture containing a substantially neutral compound having the structure:

wherein the free oxygen valances are attached to members of the group consisting of alkyl having up to about 10 carbon atoms, lower alkylene and monocyclic aryl free from substituents which are reactive in this reaction mixture, of which no more than two oxygen atoms are attached to aryl.

6. The method claimed in claim 5 wherein said polyol is at least one selected from the group consisting of neopentyl glycol, trimethylol ethane. trimethylol propane, pentaerythritol, 2-butyl-2-ethyl-l,3-propanediol and 2,2,4- trimethyl-1,3-pentanediol.

7. The method claimed in claim 5 wherein said acid is heptanoic acid, said compound is tributyl phosphate and the polyol is trimethylol propane.

8. The method claimed in claim 5 wherein said compound is at least one selected from the group consisting of tripropyl phosphate, tributyl phosphate, triamyl phosphate, phenyl dibutyl phosphate, diphenyl methyl phos phate, ethylene butyl phosphate and ethylene bis (dibutyl phosphate).

9. The method claimed in claim 8 wherein said polyol is at least one selected from the group consisting of neopentyl glycol, trimethylol ethane, trimethylol propane, pentaerythritol, 2-butyl-2-ethyl-1,3-propanediol and 2,2,4- trimethyl-1,3-pentanediol.

10. The ester product produced by the method of claim 5.

11. Method of preparing an ester of at least one alkanoic acid having an average chain length between 4 and 12 carbon atoms and trimethylolpropane which comprises reacting said alkanoic acid and said trimethylolpropane in a reaction mixture containing a substantially neutral compound having the structure:

wherein the free oxygen valances are attached to members of the group consisting of alkyl having up to about 10 carbon atoms, lower alkylene and monocyclic aryl free from substituents which are reactive in this reaction mixture, of which no more than two oxygen atoms are attached to aryl.

12. Method of preparing an ester of at least one alkanoic acid having an average chain length between 4 and 12 carbon atoms and a polyol having up to about 9 carbon atoms and one of said atoms being a quaternary carbon atom bonded to at least two methylol, said polyol being free from constituents other than said hydroxyls which are reactive in this reaction mixture, which comprises reacting said alkanoic acid and said polyol in a reaction mixture containing a trialkyl phosphate having alkyl of from one to ten carbon atoms.

13. Method of preparing an ester of at least one alkanoic acid having an average chain length between 4 and 12 carbon atoms and trimethylolpropane which comprises reacting said alkanoic acid and said trimethylolpropane in a reaction mixture containing a trialkyl phosphate having alkyl of from one to ten carbon atoms.

14. Method of preparing an ester of at least one alkanoic acid having an average chain length between 4 and 12 carbon atoms and a polyol having a quaternary carbon atom bonded to at least two methylol, said quaternary carbon atom being bonded to lower alkyl to satisfy all the valances thereof unsatisfied by methylol which comprises reacting said alkanoic acid and said polyol in a reaction mixture containing from 0.2 to 1.0 weight percent based on the theoretical weight of polyol ester formed of a substantially neutral compound having the structure:

wherein the free oxygen valances are attached to mem- -methy10lpropane ester formed of tria'lkyl phosphate having alkyl of from one to ten carbon atoms.

References Cited by the Examiner UNITED STATES PATENTS Harris 260-461 Harris 260-461 Ballard et a1. 260461 Mat-uszak et a1. 260-461 Koch 252-495 10 CHARLES B. PARKER, Primary Examiner.

MORRIS LIEBMAN, IRVING MARCUS, Examiners. 

1. AN ESTER OF AT LEAST ONE ALKANOIC ACID HAVING AN AVERAGE CHAIN LENGTH BETWEEN 4 AND 12 CARBON ATOMS AND A POLYOL HAVING UP TO ABOUT 9 CARBON ATOMS, AT LEAST ONE OF SAID CARBON ATOMS BEING A QUATERNARY CARBON ATOM BONDED TO AT LEAST TWO METHYLOL GROUPS AND UP TO 2 LOWER ALKYL GROUPS, SAID ESTER CONTAINING A MINOR AMOUNT OF THE PRODUCT PRODUCED BY THE REACTION OF SAID JPOLYOL AND SAID ALKANOIC ACID UNDER ESTERIFICATION CONDITIONS WITH A SUBSTANTIALLY NEUTRAL COMPOUND HAVING THE STRUCTURE: 